The field of the invention is patient monitoring systems. More particularly, the invention relates to a method and system that uses previous blood pressure data to determine when to change the pressure step size during blood pressure readings.
The heart muscles of humans periodically contract to force blood through the arteries. As a result of this pumping action, pressure pulses exist in these arteries and cause them to cyclically change volume. The baseline pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure. A further pressure value, known as the “mean arterial pressure” (MAP), represents a time-weighted average of the blood pressure. The systolic, MAP and diastolic values for a patient are useful in monitoring the cardiovascular state of the patient, to diagnose a wide variety of pathological conditions, and treat disease. Therefore, it is a great advantage to a clinician to have an automatic device which can accurately, quickly and non-invasively estimate these blood pressure values.
There are different techniques and devices for measuring one or more of these blood pressure values. One method in particular involves applying an inflatable pressure cuff about the upper arm of a human and inflating it above systolic pressure so as to restrict the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Korotkoff sounds, that accompany the reestablishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is an indirect measure of the systolic pressure and the pressure at which these sounds disappear is an indirect measure of the diastolic pressure. This method of blood pressure detection is generally known as the auscultatory method.
Another method of measuring blood pressure is referred to as the oscillometric technique. This method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as the patient's upper arm. The cuff is inflated to a pressure above the patient's systolic pressure and then reduced over time while a pressure sensor continues to measure the cuff pressure. The sensitivity of the sensor is such that pressure fluctuations within the cuff resulting from the beats of the patient's heart may be detected. With each beat there is a resulting small change in the artery volume which is transferred to the inflated cuff causing slight pressure variations within the cuff which are detected by the pressure sensor. The pressure sensor produces an electrical signal showing the incremental cuff pressure and a series of small periodic variations associated with the beats of a patient's heart. It has been found that these variations, called “complexes” or “oscillations,” have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure. As the cuff pressure is decreased, the oscillation size begins to monotonically grow and eventually reaches a maximum amplitude. After it reaches a maximum amplitude, the oscillation size decreases monotonically as the cuff pressure continues to decrease. Physiologically, the cuff pressure at the maximum value approximates the MAP. In addition, the complex amplitudes of cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to this maximum value. Thus, the oscillometric method is based on measurements of detected complex amplitudes at various cuff pressures.
Blood pressure measuring devices operating according to the oscillometric method are used for detecting the peak-to-peak amplitude of the pressure complexes at various applied cuff pressure levels. The amplitudes of these complexes, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressures over a range of interest. These peak-to-peak complex amplitudes define an oscillometric “envelope” and are evaluated to find the maximum value and its related cuff pressure, which is approximately equal to MAP. A cuff pressure below the MAP value which produces a peak-to-peak complex amplitude having a certain fixed relationship to the maximum value, is designated as the diastolic pressure. Likewise, a cuff pressure above the MAP value which results in complexes having an amplitude with a certain fixed relationship to that maximum value is designated as the systolic pressure. The ratios of complex amplitude at systolic and diastolic pressures to the maximum complex amplitude at MAP, are empirically derived and assume varying levels depending on the preferences of those of ordinary skill in the art. Generally, these ratios are designated in the range of 40% to 80%.
One way to determine estimates of blood pressure is to computationally fit a curve to the oscillometric envelope defined by the complex amplitude versus cuff pressure data points which are measured by a blood pressure monitor during a determination. The fitted curve may then be used to compute an estimate of the MAP value, which is approximately at the maximum value of the fitted curve and is therefore easily determined by finding the point on the fitted curve at which the first derivative equals zero. From this maximum value data point, the systolic and diastolic pressures may be computed by finding fixed percentages of the maximum complex amplitude on the curve and using the associated cuff pressure levels as the systolic and diastolic estimates. In this manner, indirect estimates of the systolic, MAP and diastolic arterial pressures may be found and ultimately output by an oscillometric device. The curve fitting technique has the value of smoothing the envelope information so that artifact variations are minimized and no single point dominates in the calculation of blood pressure, thereby resulting in more accurate estimates.
Usually, when taking an oscillometric blood pressure determination, a device will pump up to a supra-systolic cuff pressure level and take small deflation steps in order to completely measure the properties of the oscillometric envelope. However, pumping to higher than necessary cuff pressure levels and taking smaller than necessary steps may cause patient discomfort. Discomfort often results in patient motion which increases the likelihood of artifact, especially in pediatric and neonatal patients. Increased motion artifact may cause a non-determination or delay information output to the clinician. Therefore, to enhance patient comfort and reduce determination time, it is often desirable to take blood pressure readings with a minimal number of pressure steps. The oscillometric envelope pattern is simple and by judiciously choosing the particular cuff pressure levels to visit, the number of steps needed to compute an accurate blood pressure can be significantly lowered. This involves making decisions about what cuff pressure levels to visit based on the measurements and results of previous blood pressure determinations. It is generally known in the art that by visiting key points around the systolic, MAP and diastolic pressure levels, an accurate blood pressure can be estimated without the need to fill out every characteristic of the oscillometric envelope. Thus, the initial cuff pressure and size of the steps with which to deflate the cuff can be optimized if a previous blood pressure determination has been made and the blood pressure has not changed significantly. This means that the deflation steps will be much bigger than what would be used if the blood pressure were not known. Typically, these larger cuff pressure steps can be in the range of 12 to 20 mm Hg and are chosen so as to go to specific cuff pressure levels based primarily on the systolic, MAP, and diastolic pressure values of the previous determination. Since it is necessary for a patient's blood pressure to remain substantially similar to the previous determination before the accelerated inflation and deflation scheduling can be undertaken, the algorithm must have a means of guaranteeing that the blood pressure has not significantly changed. In situations where the blood pressure has significantly changed or is changing, it becomes necessary to return to a cuff deflation scheme in which the step sizes are smaller so that the details of the oscillometric envelope will be captured. Very often, these smaller cuff pressure steps are in the range of 2 to 8 mm Hg. In these circumstances, the act of returning to a different cuff pressure deflation scheme with smaller cuff pressure steps in order to obtain the full range and detail of oscillometric envelope data is called a reversion. Oftentimes, it is difficult to quickly and accurately determine when a reversion should be made. Thus, there exists a need for a method and system for quickly and effectively determining when to make a reversion to smaller cuff pressure steps and whether to increase or decrease cuff pressure during the reversion based on previous blood pressure determinations.